Two stage screw rotor machines

ABSTRACT

A two-stage screw rotor compressor in which each stage is provided with an adjustable slide valve for varying the volumetric capacity of the stage which makes it possible to vary the capacity of the compressor within wide limits while maintaining a higher efficiency within the whole operating range than in hitherto known compressors of this type. Preferably the two valves are interconnected to move in unison.

United States Patent 91 Persson et al.

[451 Sept. 4, 1973 TWO STAGE SCREW ROTOR MACHINES [75] Inventors:Walther Persson Johanneshov;

EaiiFifz neneiiaugwis a-mvnas, both of Sweden [73] Assignee: SvenskaRotor Maskiner Aktiebolag,

Nacka, Sweden [22] Filed: July 13, 1971 [21] Appl. No.: 162,244

[30] Foreign Application Priority Data July 16, 1970 Great Britain34,525/70 [52] US. Cl 418/9, 418/159, 418/201 [51] Int. Cl F0lc 1/16,F04c 17/12 [58] Field of Search 418/9, 19, 21, 201,

[56] References Cited UNITED STATES PATENTS Schibbye..l..... 418/203 XNilsson et a1. 418/159 3,432,089 3/1969 Schibbye 418/201 X 2,749,5716/1956 Pasquetti 418/9 2,645,903 7/1953 Elkins 418/21 FORE1GN PATENTS ORAPPLICATIONS 244,487 12/1960 Australia 418/201 804,327 10/1936 France418/9 819,485 9/1959 Great Britain 418/19 Primary ExaminerCarlton R.Croyle Assistant Examiner-R. J. Sher Attorney-Bauer & Goodman [57]ABSTRACT A two-stage screw rotor compressor in which each stage isprovided with an adjustable slide valve for varying the volumetriccapacity of the stage which makes it possible to vary the capacity ofthe compressor within wide limits while maintaininga higher efficiencywithin the whole operating range than in hitherto known compressors ofthis type. Preferably the two valves are interconnected to move inunison. 7

8 Claims, 3 Drawing Figures 1 TWO STAGE SCREW ROTOR MACHINES Theinvention relates to screw rotor machines having at least two stages forcompressing or expanding an elastic working fluid and being of the typein which each stage includes a pair of intermeshing screw rotors.

In for instance a two-stage compressor the first or low pressure stagedetermines the volumetric capacity of the compressor. Normally thecompressor is designed and the built-in pressure ratios of the twostages are chosen such that at full load the compressor delivers adesired gas quantity at a desired final pressure and at a satisfactoryefficiency. In almost all ranges of application the capacity of thecompressor must be variable. One way to obtain a reduction of thecapacity is to reduce the speed of the compressor. However, at lowerspeeds the unfavourable effect of the internal leakage of the stagesupon the volumetric efficiency is resulting in a reduction of theoverall efficiency of the compressor. In addition, if the compressor isdriven by an induction motor a change speed gearing must be incorporatedin the plant involving increased costs and space demands. Therefore,capacity control by speed variation is in most cases not recommendable.

US. Pat. No. 3,314,597 discloses a screw rotor compressor of the singlestagetype the capacity of which is variable by means of an axiallymovable slide valve member. In this compressor the built-in pressureratio can be kept at a suitable high level also at partial capacity andthe compressor can be adjusted down to very low capacities with a fairlyhigh efficiency.

In a screw rotor machine there is always an internal leakage which issubstantially proportional to the builtin pressure ratio orcounter-pressure. Therefore, when a single stage compressor included ina compressed air system and being of the type disclosed in theabovementioned patent is adjusted towards low capacity it will reach acondition in which its capacity corresponds to the leakage. In otherwords, when the grooves open towards the inlet port they are alreadyfilled with air of inlet pressure due to the internal leakage andtherefore no fresh air can be sucked in into the grooves. Thus, theeffective volumetric capacity of the compressor is zero in spite of thefact that it operates at full speed and the compressor can be said tofunction as a valve preventing air from leaving the system.

Two-stage screw rotor compressors are for several reasons normallydesigned such that the built-in pressure ratios of the two stages aresubstantially equal. For instance, if the total pressure ratio is 9:1each stage has a built-in pressure ratio of about 3:1 and, starting fromatmospheric pressure, the intermediate pressure will amount to about 3kp/cm at full load. This involves that the second stage need only have avolumetric capacity of about one-third of the volumetric capacity of thefirst stage in order to' be capable of absorbing the gas mass deliveredby the first stage at a pressure of 3 kp/cm.

However, if a compressor of the type disclosed in US. Pat. No. 3,314,597is used as tYe first stage in a two-stage compressor and its valvemember is moved towards low capacity position the volumetric capacity ofthe second stage remains unchanged. The second stage will thereforereduce the intermediate pressure and in certain cases the intermediatepressure may even sink below the inlet pressure of the first stage sothat this stage will act as an expander. Thus, gas will always passthrough the first stage irrespective of the position of the valve memberand therefore it is impossible to bring the compressor down to zerocapacity.

One object of the invention is to provide a multistage screw rotormachine the capacity of which may be varied within wide limits and inwhich the efficiency within the whole operating range is higher than inhitherto known machines of this type. To obtain this object the machineaccording to the invention is characterized in that each stage isprovided with adjustable valve means for varying the volumetric capacityof the stage. Preferably the movable valve members of the valve meansare interconnected to move in unison.

SUMMARY OF THE INVENTION In a two-stage compressor made according to theinvention the volumetric capacities of the first and second stages canbe reduced concurrently in such a manner that the intermediatepressureis maintained substantially constant. Therefore, in a certainposition of the valve member of the first stage the capacity of thisstage is substantially zero as explained above. In a correspondingmanner the capacity of the second stage can also be brought down tosubstantially zero. The compressor will then run at full speed at zerocapacity. This is highly desirable for instance in refrigerating plantsand compressed air systems in which the gas consumption may vary fromzero to a maximum value.

In a preferred embodiment of the machine two stages are arranged inside-by-side relationship, the rotor pairs being drivinglyinterconnected by transmission means at one end of the machine whileactuating means for the valve members are provided at the other endthereof. It in this case the high pressure port of the low pressurestage and the low pressure port of the high pressure stage are locatedat the same end of the machine as the transmission means and are incommunication with each other through a passage the valve members may beinterconnected by means of an inextensible flexible member guided byidler pulleys and passing through said passage, one of the valve membersbeing Pat. application Ser. No. 158,176, filed June 30, 1971, andassigned to the same assignee as the present'appliv cation, comprising acasing enclosing-"two working spaces disposed side-by-side and a pair ofintermeshing screw rotors in each space. Each rotor pair consists of amale rotor and a female rotor.

In the drawings numeral 10 designates the intermediate or barrel memberof the compressor casing which further includes end wall members 12 and14. In the barrel member 10 there are provided a low pressure workingspace 16 and a high pressure working space 18 disposed side-by-side andeach working space is in known manner composed of two intersecting boreswith parallel axes.

The end wall member 12 (to the left in FIG. 1) encloses an inlet chamber20 and an outlet chamber 22. The inlet chamber 20 communicates with thelow pressure working space 16 through a low pressure port 24 while theoutlet chamber communicates with the high pressure working space 18through a high pressure port 26 A transfer chamber 28 is provided in endwall member 14 and communicates with the low pressure working space 16through an outlet port 30 and with the high pressure working space 18through an inlet port 32. The outlet port 30 forms the high pressureport of the first or low pressure stage of the compressor while theinlet port 32 forms the low pressure port of the second or high pressurestage thereof.

To the outside of the end wall member 14 is secured a transmissionhousing 34 enclosing a drive gear 36 and two driven gears 38 and 40 thepitch circles being indicated by chain-dotted lines in FIG. 3. The drivegear 36 is carried by a drive shaft 42 joumalled in and projectingthrough the housing 34 and connectable to a drive motor. The driven gear38 is secured to the shaft 44 of the male rotor of the first stage andthe driven gear 40 is secured to the shaft 46 of the male rotor of thesecond stage of the compressor. For the sake of simplicity the rotorshave been omitted in the Figures.

The rotors of the low pressure stage are joumalled in combined radialand thrust bearings in end wall member 14 and in radial bearings in endwall member 12. The rotors of the high pressure stage are joumalled incombined radial and thrust bearings in end wall member 12 and in radialbearings in end wall member 14. Since the bearings are located above andbelow the plane of FIG. 1 they are not visible in this Fig.

Each stage is provided with an axially movable slide valve member of thetype disclosed in US. Pat. No. 3,314,597. The function of such a valvemember is described in detail in said specification. By moving the valvemember from one end position to the other the volumetric capacity ofeach stage can be varied between a maximum and a minimum value.

In the following discussion, identical elements 52, 54 56, $8, 60, 62and 68 in FIG. 1, elements 54 and 60 in FIG. 2 and element 68 in FIG. 3appear in two places once in the high pressure stage and once in the lowpressure stage. In the drawing, the suffix A" is added to elements inthe low pressure stage, but they are referred to herein without saidsuffix.

The valve member 48 of the high pressure stage is shown in its positionfor maximum capacity and located in an axially extending recess 50 inthe wall of the working space 18. An insert 52 in the end wall member 12forms an extension of the recess 50 so that the valve member can bemoved out into the outlet chamber 22 to its end position for minimumcapacity.

In the bottom of the recess 50 there is a groove 54 which communicateswith an oil supply opening 56. To the ends of the valve member 48 aresecured plates 58 which sealingly engage the groove 54. The valve memberis further provided with two transverse bores 60 in communication withthe groove 56. Passages 62 at the bottom of each bore 60 form nozzleopenings for injecting oil into the working space for cooling, sealingand lubricating purposes as known per se.

The valve member 64 of the low pressure stage is of the same design asvalve member 48 and need not therefore be described.

The two valve members 48 and 64 are interconnected by an inextensibleflexible member 66 such as a chain which is guided by two idler pulleys68 and passes through the transfer chamber 28 in the end wall member 14.During operation the valve members are biassed by the fluid pressuretowards their maximum capacity positions shown in FIG. 1 and the lengthof the flexible member 66 is adjusted such that in this condition theflexible member is free from slackness.

To the high pressure end of the valve member 48 of the high pressurestage is secured the piston rod 70 of a servo motor 72 of the piston andcylinder type secured to the outside of the end wall member 12. Atypical servo motor 72 is shown in US. Pat. No. 3,432,089 (especiallyFIG. 3), assigned to the same assignee as the present application. Bymeans of this servo motor both valve members 48 and 64 can be movedsimultaneously to any desired position independently of the fluidpressure acting upon the valve members the flexible member 66 beingstretched by the low pressure valve member 64 so that the valve membersmove in unison.

A compression spring 74 is inserted between the valve member 48 of thehigh pressure stage and a bracket 78 provided in thetransfer chamber 28in-the end wallmember .14. When the compressor plant is stopped thisspring moves the valve members 48 and 64 to their minimum capacitypositions thereby reducing the torque and power required to re-start theplant.

The invention is not limited to the preferred embodiment shown anddescribed but may be applied to all types of multistage screw rotormachines.

We claim:

1. In a two-stage screw rotor machine for an elastic working fluid inwhich each stage includes a pair of intermeshing screw rotors, theimprovement wherein each stage comprises adjustable valve means forvarying the volumetric capacity of the respective stage and means forconcurrently operating said adjustable valve means of each stage.

' 2. A machine as defined in claim 1, in which the ad justable valvemeans include movable valve members which are interconnected to move inunison.

3. A machine as defined in claim 1, in which the adjustable valve meanseach include an axially slidable valve member which is biassed towardsits full capacity position by the fluid pressure at the high pressureend of the respective stage, and comprising actuating means to move thevalve members to and keep them in partial capacity positions.

4. A machine as defined in claim 1, in which the two stages are arrangedin side-by-side relationship andthe adjustable valve means each includea movable valve member, and including transmission means drivinglyinterconnecting the rotor pairs at one end of the machine, and actuatingmeans for the movable valve members provided at the other end of themachine. I

5. A machine as defined in claim 4, in which the two stages are high andlow pressure stages, each having respective ports, the high pressureport of the low pressure stage and the low pressure port of the highpressure stage being located at the same end of the machine as thetransmission means and being in communication with each other through apassage, the movable valve members being interconnected by means of aninextensible flexible member guided by idler pulleys and pass- 7. Amachine as defined in claim 5, in which the flexible member comprises achain.

8. A machine as defined in claim 1, in which means are provided to movethe movable valve members towards the low capacity position when themachine is stopped.

* I F l v UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. $756,753 Dated September 4, 1973 Inventor(5) Walther PERSSON and Lauri tz Bnedictu SCHIBBYE It is certified that error appears in theabove-identified patent v I and that said Letters Patent are herebycorrected as shown below:

On initiel page of patent after "[75] Inventors;

Walther Pers son," Johanneshbvf' 'chenge 5 Y Laur-itz 'Benedi'ctus" to--Lauritz :Benedictusschibbye I Signed and seeled. t his 18thd a'yiofDecember T1973.

(SEAL-)7 Attest:

EDWARD M. FLETCHER, JR, RENE D,.TEGTDEYER Attesting Officer ActingCommissioner of Patents

1. In a two-stage screw rotor machine for an elastic working fluid inwhich each stage includes a pair of intermeshing screw rotors, theimprovement wherein each stage comprises adjustable valve means forvarying the volumetric capacity of the respective stage and means forconcurrently operating said adjustable valve means of each stage.
 2. Amachine as defined in claim 1, in which the adjustable valve meansinclude movable valve members which are interconnected to move inunison.
 3. A machine as defined in claim 1, in which the adjustablevalve means each include an axially slidable valve member which isbiassed towards its full capacity position by the fluid pressure at thehigh pressure end of the respective stage, and comprising actuatingmeans to move the valve members to and keep them in partial capacitypositions.
 4. A machine as defined in claim 1, in which the two stagesare arranged in side-by-side relationship and the adjustable valve meanseach include a movable valve member, and including transmission meansdrivingly interconnecting the rotor pairs at one end of the machine, andactuating means for the movable valve members provided at the other endof the machine.
 5. A machine as defined in claim 4, in which the twostages are high and low pressure stages, each having respective ports,the high pressure port of the low pressure stage and the low pressureport of the high pressure stage being located at the same end of themachine as the transmission means and being in communication with eachother through a passage, the movable valve members being interconnectedby means of an inextensible flexible member guided by idler pulleys andpassing through said passage, one of the movable valve members beingconnected to said actuating means.
 6. A machine as defined in claim 5,in which said actuating means comprises an hydraulic servo motor of thepiston and cylinder type, the piston thereof being operatively connectedto the movable valve member of the high pressure stage.
 7. A machine asdefined in claim 5, in which the flexible member comprises a chain.
 8. Amachine as defined in claim 1, in which means are provided to move themovable valve members towards the low capacity position when the machineis stopped.